This invention relates to systems for transmission and reception of digital video over packet switched digital networks.
Packet switched digital networks are used to transfer packets of digitized data among users coupled to the network. Data to be sent over the network, from a source device to a receiving device, is typically combined with a header containing an address to form a data packet for transmission. The address portion of the packet directs the packet to the desired receiving device on the network to establish a virtual communications channel, or digital end-to-end connection, between the source device and the receiving device. The data portion of the packet is formatted to correspond to the requirements of any one or more of the services available on the network.
Traditionally, networks were customized to specifically accommodate only certain types of services, which lead to a variety of incompatible, service-oriented communications networks. In recent years, the CCITT has adopted a series of standard multi-purpose user network interfaces (UNI) for an integrated services digital network (ISDN) supporting a wide range of voice and non-voice services over a digital end-to-end connection. The CCITT has considered a number of broadband network interface approaches supporting a wide range of data, voice, and video services, including Asynchronous Transfer Mode (ATM), which stands out among the other approaches. ATM is a connection oriented transfer technique, where fixed-size ATM data packets (cells) from different sources are asynchronously multiplexed onto a communications channel, providing efficiencies in bandwidth utilization and allocation of variable bandwidths to different services.
Video services vary greatly in their bandwidth and image resolution requirements. For instance, video telephone service requires relatively little bandwidth compared to the bandwidth requirements for high definition television (HDTV) services. Data compression is necessary to minimize the bandwidth requirements for all video services on a network, especially where network congestion is anticipated. Furthermore, the differing image resolution requirements of the various video services creates a compatibility issue among services desiring to use the same video information transmitted on the network. A compatible video encoding scheme is necessary to provide data compatibility among the video services.
Packet switched networks may experience data loss, due to, for instance, data buffer overflows or errors in the packet headers. Data loss typically affects the video quality of the transmitted images in various ways, depending on how the video image is encoded and packaged into ATM data packets. Robust video encoding techniques are required to minimize the effect of lost data on the quality of video images sent across the network.